1. Technical Field
The present intention relates to a recording medium and an image recording set,
2. Background Art
In recent years, electronic paper has been put into practical use and gained attention because it is visible, portable, thin, and lightweight like paper material requiring no backlight and less power consumption.
Electronic paper is classified into internally-driven paper-like display (hereinafter referred to as PLD) that includes a drive element inside and can display images by itself and externally-driven rewritable paper using a recording medium on which writing is performed by a printer.
The rewritable paper includes all recording media such as paper and sheets on which rewriting can be repeatedly performed, and is synonymous with a rewritable recording medium included in the present invention.
Various systems of electronic paper have been proposed. Of these, as a PLD, an electrophoretic system, that performs display by changing an optical reflectance by causing white and/or colored particles charged in a liquid to migrate to the surface side or the opposite direction of a display medium by an electric field is known as the most practical system.
As an example of the electronic paper of such system. JP-S50-15115-B discloses a display or recording device, in which an electric field is applied to a mixed multi particle dispersion containing particles that migrate in opposite directions depending on the electric field, to optically shield certain particles, to thereby change optical reflection characteristics of the dispersion system.
In addition, JP-2551783-B1 (JP-H01-086116-A) discloses an electrophoretic display device, in which a large number of microcapsules each encapsulating a dispersion system obtained by dispersing electrophoretic particles in a colored dispersion medium is provided between a pair of counter electrode plates.
Furthermore, JP-4410135-B1 (JP-2005-227795-A) discloses an ink, in which an electric field is applied to microcapsules each containing first particles and second particles having reverse polarity to that of the first particles to thereby migrate either the first or second particles to the surface.
As described above, the electrophoretic system, particularly a microcapsule electrophoretic system, has been already been put into practical use as electronic paper, and has gained much attention.
Meanwhile, JP-3680996-B1 (JP-2003-526817-A) discloses a method, in which appropriately regulated cells, so-called micro cups, are provided on a film, and a dispersion system containing electrophoretic particles is encapsulated in each cell.
This method includes forming a layer of a precursor of a thermoplastic material or thermoset material, conducting an emboss process with embossing, and then curing. Since the production can be carried out with a roll-to-roll process in this method, this is an excellent method in view of low cost production.
Such electrophoretic electronic paper is capable of retaining displayed content without requiring electric power after displaying the content once, which is recognized as one of the advantages. However, in fact, it is not possible to practically retain images because image density changes with time unless an electric field is applied or image quality deteriorates by static electricity and pressure from outside.
In attempts to solve these problems, for example, JP-2007-286124-A discloses a method of controlling a solubility parameter in a dispersion medium by making walls of microcapsules from a grail copolymer. Also, JP-2010-002933-A discloses a method of dissolving or dispersing a polymer that does not become thick in a fluid suspension.
These technologies are appropriate to improve the image retention (retentiveness) required for PLD.
However, these are not sufficient to retain images on rewritable paper.
This difference lies in the structures and the purposes of use of PLD and rewritable paper.
PLD has substrates each having an electric conductive layer, as electrodes, at the bottom and top of a display layer in which electrophoretic particles move and is covered by a housing, so that it is never or little affected from the outside. In addition, a voltage can be applied thereto if desired, which makes it possible to retain an image on PLD.
However, rewritable paper is not covered by a housing and is easily affected from the outside. In addition, a recording medium includes no element or power source to apply a voltage, meaning, that retaining an image for a long period of time is impossible from outside.
It is possible to reduce at impact from outside by providing a protective layer to cover rewritable paper in some cases. However, this is not sufficient to enhance g image retention.
Considering that PLD is used for display, images are frequently rewritten and demand for image retention is not strict. By contrast, rewritable paper is required to retain records for a long period of time almost permanently, like paper.
As a technology to significantly enhance image retention, there are a number of methods including controlling the viscosity of a dispersion medium in which electrophoretic particles are dispersed, lowering the viscosity by heating to conduct recording, and thereafter fixing colored particles cooling-down.
For example, JP-S50-15115-B1 mentioned above discloses a method of using a resin, rubber, waxes, or synthetic waxes that are solid at normal temperatures and softened thermally or chemically.
JP-4168556-B1 (JP-2001-147451-A) discloses a method of forming a uniform liquid phase by dissolving a solid phase of a dispersion medium, which has solid and liquid phases present at room temperature, during electrophoretic migration in a liquid dispersion containing electrophoretic particles, the dispersion medium a dye, and a dispersant.
JP-2001-301325-A discloses a method in which a heat-meltable substance that is solid when it is not heated and becomes liquid when it is heated, specifically, waxes, saturated fatty acid, higher alcohols, and electrophoretic fine particles are encapsulated into microcapsules.
JP-2002-365670-A discloses a method using a mixture that has fluidity at an electrophoretic operating temperature, and becomes an optically opaque solid at a temperature lower than the electrophoretic operating temperature, specifically, a mixture of a substance selected among long-chain alcohols, phenols, aromatic ketones, decanes, and dodecanes, and a substance selected among paraffin, solders, waxes, and saturated hydrocarbons.
JP-2003-91022-A discloses a method using higher paraffin hydrocarbons, solders, waxes, or aromatic hydrocarbons as a dispersion medium that is solid at normal temperatures and becomes liquid by heating.
JP-2007-140367-A discloses a method in which a dispersion medium is composed mostly of rice wax and contains a surfactant.
Thus, a dispersion medium that is solid at normal temperatures and becomes liquid when being heated is very effective for improvement in image retention because electrophoretic particles are fixed by cooling after recording.
However, since these technologies use waxes and saturated aliphatic acids, the dispersibility of electrophoretic particles significantly deteriorates when compared to a case where a solvent having excellent dispersability is used as the dispersion medium. For this reason, electrophoretic properties may be impaired, or image quality deteriorates due to aggregation of particles.
Further, temperature responsiveness is low in transition between the solid state and the liquid state of electrophoretic particles, which causes problems such that rewriting speed decreases, the viscosity of a dispersion medium is not sufficiently lowered even when heated, thereby degrading contrast, and the quality of image deteriorates over repeated rewriting.
A method of using a dispersion medium that turns into a gel in a PLD is disclosed.
For example, JP-H02-196227-A discloses a thermal writing system which includes a dispersion medium that turns into a gel and a sol (liquid) reversibly depending on temperatures and electrophoretic particles and locally causes the electrophoretic particles to migrate by locally converting the dispersion medium into a sol.
However, with this technology, as a dispersion medium that turns into a gel and a liquid, specifically, is solvent is used for a high-molecular weight substance, resulting in a poor solvent. This high-molecular weight substance is dissolved when the temperature of the solvent is high and the high-molecular substance becomes insoluble and turns into a gel when the temperature is low, so that the high-molecular substance itself does not have thermal reversibility.
Therefore, the temperature responsiveness is low in transition between a gel and a liquid and liquefaction is local, so that display speed and image quality are not satisfactory.
JP-2003-149691-A discloses a display gel for electrophoretic display, containing a dispersion medium, white particles, colored particles, and a gelling agent.
However, in this technology although a gelling agent is contained in the dispersion medium, thermal reversibility is not described. Actually, due to gelatinization, the dispersion medium is substantially equally divided into continuous and/or discontinuous minute regions and turns into a gel and the particles electrophoretically migrate by the action of an electric field in the dispersion medium in a gel state.
Therefore, rewriting speed and contrast remain unsolved.
JP-2007-11342-A discloses a display medium for electrophoretic display containing a set of a liquid system, a thermoreversible gelling agent, and colored particles for PLD. With this technology, by containing the thermoreversible gelling agent, the display medium turns into non-gel and is reduced in viscosity at temperature close to or higher than a melting point of the gelling agent, so that images can be limited in a display device.
When the display device is cooled after an image is formed, the medium turns into a gel, so that the image is stably maintained unless an electric field is applied.
In the method using a thermoreversible gelling agent, the gelling agent itself causes gelatinization, so that the temperature responsiveness is higher and more stable than in the conventional technologies in which waxes and solders are added or gelatinization is controlled by solubility of a high-molecular substance in a solvent, so that this method is very excellent for enhancing image retentiveness.
However, as long as the thermoreversible gelling agent is also controlled by temperature, the influence of reduction in rewriting speed on use with frequent image rewriting increases.
As described above, this technology using a thermoreversible gelling agent is premised on that it is used for a display device having two parallel transparent conductor electrode panels, that is, a PLD, so that the disadvantage of deterioration in display responsiveness becomes greater than the advantage of semipermanent image retentiveness by using a thermoreversible gelling agent.
In addition, a heating device and a cooling device must be provided inside the device, so that the thickness and weight of the PLD increase, and this is also a big disadvantage.
Therefore, the effect of use of the thermoreversible gelling agent has not been sufficiently utilized, and has not been applied to a PLD in actuality.
It is considered that the need for image retentiveness required for rewritable paper is higher than that for PLD as described above.
In order to perform rewriting, the PLD is configured as a device, so that the rewriting frequency is high.
Therefore, it does not necessarily require semipermanent image retentiveness.
On the other hand, the rewritable paper is rewritable, however, on the assumption that it is used instead of paper, high image retentiveness is required.
In addition, rewritable paper that is not covered by a housing is more easily affected by static electricity and pressures front outside than the PLD covered by a housing. This creates a problem when enhancing image retentiveness.
For this reason, image retentiveness is a large issue for rewritable paper rather than for PLD.
In addition, in a case of controlling transfer of sol-gel of a dispersion medium contained in an image recording layer by heating, heat conductivity of substrates or a protective layer to the image recording layer is also an issue.
If heat conductivity of substrates or it protective layer to the image recording layer is extremely decreased, a variety of problems occur such that solation is not sufficient, resulting in decrease of contrast, gelation does not occur instantly, thereby degrading image quality, or recording speed is reduced.
As described above, an image recording layer having a quick temperature responsiveness, which is formed using a thermoreversible gelling agent, is affected by a substrate and a protective layer, etc. formed to cover the layer. As a result, contrast deteriorates, visibility becomes interior, or heat conductivity becomes low which has a negative impact on printing speed.